When starting a project, it is often necessary to begin by installing a foundation constituted by bored piles made in the ground where the project is to be built. That technique consists in drilling a borehole in the ground of diameter matching that of the pile that is to be made, and then filling the borehole with concrete or group or some other suitable material.
There also exist so-called tunneling machines having a rotary head that are used to make tunnels that are horizontal or substantially horizontal, and that might pass through rock that is very hard, such as granite. Nevertheless, it will be understood that the conditions under which vertical boreholes are made are completely different from those encountered when digging a tunnel.
While drilling a borehole, the drilling machine is immersed in a fluid that fills the borehole. In addition, in order to reach the layer of hard rock, it is often necessary to drill to great depth, e.g. About 15 meters (m) to 50 m through layers that are softer. During this operation, as the borehole advances, it is necessary to install successive casing elements or to maintain the excavation by means of an appropriate drilling fluid.
It will also be understood that the rotary head cutting machine must be capable of developing very high levels of thrust, e.g. Of the order of 500 (metric) tones (t), together with a very high level of torque when drilling into granite.
It is therefore necessary to be able to take up said thrust and said torque as developed by the cutting machine at a depth which can be quite considerable, e.g. 15 m to 50 m, and in a space that is full of water, bentonite mud, or an appropriate drilling fluid.
It can clearly be seen that the techniques for taking up thrust and torque that are appropriate for use with conventional tunneling machines are unsuitable for use in conditions corresponding to drilling boreholes to make bored piles.
It should also be added that “large-diameter bored piles” means piles having a diameter of about 1.5 m to 4 m. This size is very different from that which is encountered when making a tunnel.